The present invention relates to a color television camera apparatus of the type in which a video signal level is automatically adjusted.
In a color television camera apparatus, a color image is separated, by an optical system, into three primary color components which in turn are directed to image pick-up tubes. Analog video signals produced from the image pick-up tubes are applied to process amplifiers for various adjustments, respectively. The output signals from the process amplifiers are applied as camera output signals to the next stage circuit, for example, a color coder. The process amplifier includes various adjusting circuits. The amount of adjustment in the adjusting circuit can manually be set at a proper value by a camera man, while seeing a monitor. Recently, such adjustment has automatically been made. For the automatic adjustment, the output signals from the process amplifiers are applied through A/D converters to an adjustment control circuit containing a microcomputer where the adjusting amount is determined. Control signals produced from the adjustment control circuit are supplied to the control terminals of the process amplifiers, through D/A converters.
For distributing the output signal from the process amplifier to a plurality of circuits such as the color coder and the adjustment control circuit, the output signal from the process amplifier is supplied to a distributing circuit. The distributing circuit is comprised of an amplifier with almost a zero output impedance which is supplied with the output signal from the process amplifier and a plurality of output resistors each having one of their ends connected to the output terminal of the amplifier. The other end of each of the output resistors are respectively connected to the color coder and the adjustment control circuit, etc. In fact, the connection is performed through a cable, normally, a coaxial cable. The output resistance of the distributing circuit is set equal to the characteristic impedance 75.OMEGA. of the coaxial cable for impedance-matching of the distributing circuit of the coaxial cable. The input terminal of the A/D converter connected to the input of the adjustment control circuit and the input terminal of the color coder are terminated by the termination resistor of 75.OMEGA. equal to the characteristic impedance of the coaxial cable. The terminal voltage of the termination resistor is applied as a video signal to the color coder and the adjustment control circuit.
It is necessary that the output resistance of the distributing circuit and the input resistance of the color coder and the adjustment control circuit must accurately coincide with the characteristic impedance of the coaxial cable. Actually, however, it is almost impossible to obtain the accurate coincidence of the impedances. Thus, the connection is always accompanied by a slight amount of impedance mismatching. Therefore, there frequently occurs a difference between the input levels to the color coder and the adjustment control circuit although the same video input signal is applied to them. A difference between the level-down of the transmission cables from the distributing circuit to the color coder and to the adjustment control circuit possibly causes such input level difference between both circuits. This results in inaccuracy of the adjustment. This indicates that in the case of the color cameras, the input level difference is caused for every primary color pick-up tube. Accordingly, it is impossible to obtain a balance among the three colors.
The above discussion has been made using the case of a single camera. Practically, a plurality of television cameras are used when the broadcasting program is processed. In this case, the adjustment must be performed for all the cameras. It is impossible to accurately make the output video levels of the cameras coincident with the input level of a single adjustment control circuit.
In addition to the above problems, the prior art further involves the following problems. The process amplifier performs the analog operations of addition and multiplication. Therefore, it is affected by variations of the ambient temperature and the power voltage. To solve this problem, it is necessary to increase the warm-up time of the camera. More specifically, one to two hours is taken for the warm-up time and then the automatic adjustment is performed. Also after the automatic adjustment, a level drift takes place in the adjusting circuits of the process amplifier, as the result of the analog processing. For cancelling the time varying drift, the automatic adjustment is again needed. The use of the D/A converter leads to the increase of the manufacturing cost, and causes the level drift. Additionally, when the output signal from the process amplifier is distributed to the adjustment control circuit and the color coder and the A/D conversion is performed in the adjustment control circuit, the LSB in the A/D converter is indefinite, resulting in an error of .+-.1/2 LSB with respect to the video signal to the color coder. As a result, there occurs a difference between the A/D converter video signal applied to the adjustment control circuit and the video signal applied to the color coder.